The use of different materials in high temperature environments is required in a variety of aerospace, nuclear reactors, solar energy and other applications. Thermal analysis is an important tool in selecting and testing various materials for this use and radiometric measurements are an essential part of a complete thermal analysis of a material. Commercially available infrared imaging systems are frequently employed for performing radiometric measurements. For absolute radiometric analysis, knowledge of the radiant properties of the surface under investigation is required. For opaque materials, the knowledge of surface emittance is sufficient, because the reflectance can be inferred from the emittance using Kirchhoff's law. In general, the emittance of a surface is a function of wavelength, temperature, direction and surface conditions, such as roughness, oxide layers, physical and chemical contamination and, in the case of dielectric materials, the grain structure.
When specifying emittance, the wavelength and direction at which emittance is obtained needs to be specified. Monochromatic or spectral emittance is used to indicate emittance at a certain wavelength, while total emittance implies emittance integrated over all wavelengths. Directional emittance implies emittance in a specific direction, while hemispherical emittance implies emittance integrated over the entire hemispherical space. The monochromatic hemispherical emittance is the ratio of the hemispherical emittance of the surface to the hemispherical emittance of a black body at the same wavelength and temperature. The monochromatic hemispherical emittance decreases with increasing wavelength for metals, and generally increases with increasing wavelength for electric nonconductors. For metals, the monochromatic hemispherical emittance increases with increasing temperature and is approximately proportional to the square root of the absolute temperature.
For electric nonconductors, the variation of monochromatic hemispherical emittance with temperature is not very clear, but there is evidence that the emittance varies very slowly with temperature. The total hemispherical emittance is obtained by integrating over all wavelengths from zero to infinity. The directional angular emittance is defined as the ratio of emitted intensity in a specific direction to the intensity of black body radiation at the same temperature. The distribution of emittance for most surfaces is generally dependent upon the inclination angle and the angle of rotation, while for isotropic surfaces, emittance only varies with inclination angle. A diffuse surface is defined as a surface whose emittance is uniform in all angular directions. A black body is a diffuse emitter of radiant energy. Two additional qualifiers are needed for reflectance. A diffusely reflecting surface reflects a single incident ray over all angles with uniform intensity. A specular reflector reflects a single incident ray as a single ray at a reflection angle equal to the incidence angle. A surface generally behaves specularly when the surface roughness is very small compared to the wavelength of incident radiation.
There are extensive tabulations of total hemispherical and total normal emittance data in the literature but there are some discrepancies between published data mainly due to variations in surface conditions. Also, the majority of published data give total emittance, while for radiometry using bandwidth limited infrared imagers, the emittance needs to be integrated over the bandwidth of the infrared imager. To complicate matters further, most infrared imagers do not have a uniform spectral response throughout their system before being integrated over the desired bandwidth. Therefore, for accurate radiometric work, it is necessary to measure the spectral hemispherical emittance of the surface under investigation in the bandwidth of the infrared imager, then weigh the data with respect to the relative spectral response of the imager. The data is then integrated over the bandwidth of the imager. The relative spectral response of the imager may be measured with a spectroradiometer/monochromater system or may be supplied by the Infrared Imager manufacturer.
Some applications require infrared imaging of targets with surface curvature. If the purpose of the measurements is to obtain an average surface temperature, then the hemispherical emittance will be sufficient. But if detailed surface temperature variations on curved surfaces are required, then the directional variation of emittance for the surface material is needed. Experimental data for variation of directional radiation properties are limited but the electromagnetic theory provides the approximate variation of the directional emittance of metals and electric nonconductors under special conditions. Since most applications of radiometric infrared imaging in aerospace research involves targets with surface curvature, a simple measurement technique for accurately measuring the variation of directional emittance of surfaces using infrared imaging systems would prove a valuable research tool.
It is therefore an object of the present invention to provide an apparatus for measuring the variation of directional emittance of a surface using infrared imaging system.
Another object of the present invention is a process for measuring the variation of directional emittance of a surface at various temperatures with an infrared imaging system.
A further object of the present invention is a process of measuring the directional emittance of flat surfaces at various angles to simulate curvatured target surfaces.